1. Field of the Invention
The invention relates to a process for the depolymerization of one or more polyamides into monomeric components in the presence of water by means of at least one alkali metal compound, at least one alkaline-earth metal compound or a mixture thereof.
2. Description of Related Art
Such a process is known from U.S. Pat. No. 5,359,062, which describes a process in which nylon-6 is depolymerized by means of sodium hydroxide. However, this depolymerization takes place at a pressure of more than 2 MPa. The conversions achieved are relatively low. In the example, conversion of nylon 6 to caprolactam of only 65% is achieved during 5 hours at a pressure of 5 MPa.
The reprocessing of polyamides (production waste for instance) has taken place for about 40 years, and is carried out in particular at polyamide producers and polyamide fiber spinners. In practice, this product reprocessing still takes place by means of the so-called phosphoric acid route. This means that the polyamide is depolymerized by means of steam with phosphoric acid as the catalyst, as described in, for instance, U.S. Pat. No. 3,182,055. The drawback of this route is, however, that phosphate-containing production waste is formed, which in turn must either be worked up or be disposed of as waste.
Depolymerization of polyamides is relevant for the reprocessing, for example, fibers, films, chips and, injection molded or extruded polyamide products. Of course, the commercial feasibility of reprocessing polyamide-containing waste is directly dependent on the economic/technical possibility of converting the polyamide component of this waste into monomeric components which can, preferably, without further conversion reactions, be reused. For nylon-6 the monomeric components are caprolactam and caprolactam precursors. Precursors are understood to be those compounds which can be used for the preparation of the polyamides without first having to be converted into caprolactam. An example of a precursor is amino caproic acid. In the case of nylon-6,6 the monomeric components are hexamethylene diamine and adipic acid.
Examples of polyamides which can be depolynurized according to the invention are nylon-6, nylon-6,6, nylon-4,6, nylon-12, nylon-4,12, nylon-6,12, nylon-8, etc. In this patent application, polyamides are understood to be both polymers and oligomers of amides. Oligomers are understood to be both linear and cyclic oligomers of, for instance but not limited to, caprolactam. They are also understood to be polyamide-containing mixtures. The mixtures may also include compounds such as, for instance, colorants, UV stabilizers, matting agents, antistatic agents, impact modifiers, heat stabilizers, dirt-repelling additives, spinfinish oil, and other additives.
There is, therefore, a need for a process for the depolymerization of one or more polyamides into its monomeric components with a higher and faster rate of conversion without the environmental hazards of the prior processes.